Generations of biochemistry students have been taught that the Kreb’s cycle, or tricarboxylic acid (TCA) cycle, serves a dual purpose: to produce energy for cells and to form the fundamental components for growth. However, researchers are uncovering that metabolic pathways – including well-established ones like the TCA cycle – can actually be assembled in diverse manners, taking on many more functions than previously perceived. Recent discoveries reveal that the TCA cycle has another often-overlooked function: waste disposal.
Cells adapt their nutrient metabolism based on their type and developmental stage. For instance, researchers found a few years back that during infections, immune cells adjust their TCA cycle to synthesize itaconate, an antimicrobial metabolite. Lydia Finley, a cancer researcher at Memorial Sloan Kettering, and her team were curious about the metabolic activities in various cell types. ‘We understand that requirements vary depending on the cellular identity,’ Finley states. ‘What are the various methods cells use to manage metabolism, and what role does metabolism play for them?’
To explore this question, Finley’s team disabled one of the enzymes in the TCA cycle, creating a bottleneck that led to the buildup of citrate, the starting metabolite in the cycle. They discovered that this buildup indicates an influx of nutrients that exceeds demand, prompting a stress response. Subsequently, when they eliminated a second enzyme in the TCA cycle – the one responsible for producing citrate – the cells recovered and grew normally despite the disruption of energy generation through the TCA cycle. This outcome highlighted that the ability to prevent citrate accumulation, by either clearing it rapidly or not producing it initially, was vital for maintaining cellular health.
In mice with this TCA cycle mutation, the kidney was the first organ to deteriorate. This is due to the fact that the kidney uniquely utilizes citrate as fuel: it thus encounters the most significant issues when citrate clearance fails. Surprisingly, despite a dysfunctional TCA cycle, the heart and brain, which demand substantial energy, remained unaffected for three weeks, the duration of the study. This indicates that cells can readily seek alternative metabolic pathways for energy production. The essential role of the TCA cycle was its function in citrate clearance. ‘Essentially, it acts as a garbage compactor for the cell,’ Finley explains.
‘We usually consider the crucial aspect of a metabolic pathway to be the generation of the end product,’ remarks Jared Rutter, a biochemist at the University of Utah, who was not part of the research. ‘It’s a surprising finding that the most harmful effect of a metabolic pathway obstruction isn’t blocking the production of the final product, but rather blocking it mid-process and causing a buildup of a toxic intermediate. This study illustrates that concept in a mechanistic detail that is infrequently achieved.’
Finley’s research suggests a much wider phenomenon with clinical relevance. Genetic metabolic disorders can lead to rare diseases when patients have germline mutations in metabolic enzymes. Sometimes, conditions arise because defective enzymes in a metabolic pathway hinder the production of downstream products. However, Finley’s findings imply that patients might often suffer due to the accumulation of toxic intermediates.
‘This idea might not be fully recognized outside the aficionados of this field,’ Rutter observes. Referring to the findings, he mentions, ‘the implications extend far beyond citrate, and there are likely numerous examples of this yet to be discovered in the future.’